Beginning with the 1994 model year, automotive manufacturers began the implementation of systems designed to diagnose malfunctioning emission related components on-board the vehicle. The set of regulations requiring these on-board diagnostic capabilities is commonly referred to as either OBD-II (On-Board Diagnostics-II for California) or OBD (On-Board Diagnostics for the remaining 49 states). One component which the on-board diagnostics system must diagnose is the catalytic converter. The Federal OBD regulations for catalytic converter efficiency monitoring require that the Malfunction Indicator Light (MIL) be illuminated when catalyst performance degrades to a level such that the Federal Test Procedure (FTP) tailpipe HC emissions increase by more that 0.40 gm/mi relative to the baseline emission level or before tailpipe HC emissions exceed 0.60 gm/mi. The California requirements for low emission vehicles (i.e. TLEVs, LEVs, and ULEVs) specify that catalyst malfunction is to be determined when either of the following occurs: 1) tailpipe HC emissions exceed 1.75 times the applicable non-methane organic matter (NMOG) standard, or 2) non-methane hydrocarbons (NMHC) conversion efficiency of the monitored portion of the catalyst system falls below the 50% level.
Over the past 5-6 years, the automotive industry has deployed significant resources toward the development and implementation of the various monitors associated with OBD (Federal) and OBD-II (California). In addition to these large investments, every automotive manufacturer must continue to provide for considerable expenditures in engineering and testing resources for the calibration of the various monitors for each of their particular vehicle engine families. For example, any modification to the base engine hardware, engine operating strategy, or engine calibration effecting emissions would generally necessitate recalibration of the catalyst monitor for a given vehicle. Also, changes to the exhaust system configuration, catalyst placement, or catalyst washcoat formulation require that the monitor be recalibrated. In most cases, last minute changes or running changes to the base engine hardware or calibration along with the exhaust and catalyst system are generally precluded due to the prohibitive resource requirements associated with recalibration of the effected OBD monitors.
Calibration of the catalyst monitor requires the use of a threshold converter or catalyst hardware. As used herein, the terms "converter" and "catalyst" are used interchangeably. A threshold converter or catalyst is one whose performance or activity is degraded to a point which causes FTP HC tailpipe emissions to be equal or very close to the level where the MIL is to be activated.
The inventors of the present invention have found that the availability of threshold converters or catalysts to support catalyst monitor calibration has been a problem. The use of field aged converters to satisfy the need for the threshold hardware is not feasible due to the scarcity of in-use converters with threshold efficiency levels because a threshold converter is intended to represent a catastrophic failure which is expected to occur on only a very small fraction of the vehicle population. Additionally, catalyst washcoat technology has evolved at a considerable pace in recent years. The earlier generations of catalyst technology which are currently in service are significantly different compared to the catalyst technology which most manufacturers are planning to use in the very near future and are therefore inappropriate for catalyst monitor calibration purposes.
To simulate a field aged catalyst, prior methods include placing a catalyst on engine dynamometers and inducing various levels of misfire in order to degrade catalyst performance to the required threshold level. The inventors of the present invention have found various disadvantages with this approach. For example, under steady state conditions, it is difficult to achieve the necessary midbed temperatures to degrade the catalyst to the desired efficiency level within a reasonable time. Also, considerations about the availability of engine dynamometers and other resources required to meet the demand for threshold catalyst hardware with the misfire method may be limited.
The preferred method of producing threshold catalysts is oven aging. Here, the catalyst bricks are removed from the converter shell assembly and baked in an oven at temperatures ranging between 1000 and 1350.degree. C. and times from 2 to 32 hours. After aging, the bricks are recanned and installed onto the vehicle. The vehicle is then usually driven for a few hundred miles to provide for a stabilization or break-in period prior to emission testing. One drawback associated with this method is the difficulty associated with determining the appropriate aging time and temperature required to degrade the catalyst performance to a specified level for a given vehicle. As a result, several iterations are often necessary in order to obtain the desired catalyst performance level. Once the emission results are obtained for one given aging, either the aging time or temperature is accordingly adjusted in order to move the tailpipe emissions closer to the desired threshold level. Generally, anywhere from 2 to 5 iterations can be required in order to obtain a threshold catalyst system. This type of trial and error procedure results in significant time delays and inefficiencies in the OBD calibration process. In addition, the need to place the catalyst on a vehicle for a predetermined drive cycle to stabilize the activity of the catalyst further increases the time and resource requirements for calibration of the monitor.